1. Field of the Invention
The present invention relates to a solder joint structure suitable for use in lead-free soldering of an electronic component having a low heat resistance and to a method of soldering electronic components.
2. Description of the Related Art
In soldering terminals of various electronic components onto solder lands of print circuit boards, a Sn—Pb eutectic solder, e.g., 63Sn-37Pb solder, has been widely used. As the use of lead-free solder materials increases to avoid environmental pollution, various proposals of solder materials that meet the demand for lead-free soldering are made. Among such proposals, Sn—Ag—Cu solder materials are drawing much attention since they have superior thermal fatigue characteristics and creep property (e.g., Japanese Unexamined Patent Application Publication No. 2002-158438). Whereas Sn—Pb eutectic solder melts at 183° C., the melting point of Sn—Ag—Cu solder materials is higher, i.e., approximately 220° C. Accordingly, Sn—Ag—Cu solder materials are not preferred in soldering electronic components having relatively low heat resistance. For example, there are a large number of lead-terminal-mount electronic components that cannot withstand temperatures above approximately 200° C. The terminals of such electronic components cannot be soldered with Sn—Ag—Cu solder materials having the melting point of approximately 220° C.
Meanwhile, a tin-zinc (Sn—Zn) solder material has been known as a lead-free solder material suitable for soldering thermolabile electronic components (e.g., Japanese Unexamined Patent Application Publication No. 2002-66783). The Sn—Zn solder material is an alloy prepared by adding 8 percent by weight of zinc and 3 percent by weight of bismuth to tin, i.e., a Sn-8Zn-3Bi alloy. Since, the melting point of the alloy is low, i.e., approximately 200° C., the molten Sn—Zn solder can be joined with the terminal of the thermolabile electronic component without inflicting any problem. However, a temperature cycling test in which the Sn—Zn solder material is applied on a patterned conductor containing copper, e.g., a copper foil, reveals that the joint strength decreases due to the interaction between copper and zinc and that the reliability of soldering is seriously degraded as a result. In other words, required reliability cannot be achieved if soldering is performed by directly applying the Sn—Zn solder material on a solder land, i.e., a patterned copper foil, on a print circuit board.
In order to overcome this problem, as shown in FIG. 7, a method whereby underlayers, namely, a nickel plating layer 3 and a thin gold plating layer 4, are formed on a patterned copper foil 2 of a print circuit board 1 before performing low-melting-point soldering on the gold plating layer 4 using a Sn—Zn solder 5 has been employed. The nickel plating layer 3 prevents zinc from diffusing into the patterned copper foil 2. The gold plating layer 4 covers the nickel plating layer 3 having poor solder wettability to secure joint with the Sn—Zn solder 5. A terminal hole 6 is formed in the patterned copper foil 2 on the print circuit board 1. After the Sn—Zn solder 5 is applied on the gold plating layer 4, a lead terminal 31 of a thermolabile electronic component is inserted into the terminal hole 6 and is heated in a reflow furnace at approximately 200° C. so as to connect the lead terminal 31 to the Sn—Zn solder 5 by fusion bonding.
According to this solder joint structure in which the Sn—Zn solder 5 is formed on the patterned copper foil 2 with the nickel plating layer 3 and the gold plating layer 4 therebetween, a decrease in joint strength resulting from the interaction between copper and zinc can be avoided, thereby achieving reliable soldering of thermolabile electronic components using lead-free solder materials.
The conventional solder joint structure shown in FIG. 7 simultaneously achieves reliable soldering of thermolabile electronic components while avoiding environmental pollution resulting from use of lead and reliable soldering of thermolabile electronic components. However, since the nickel plating layer 3, which has poor solder wettability, must be coated with the gold plating layer 4, material cost is high, and the process requires extra steps. Thus, increased cost has been a problem.